Field of the Invention
The present invention relates to a contra-rotating propeller system for an aircraft turbine engine.
The invention applies to aircraft turbine engines, e.g. turbojet engine or turbo-prop. More particularly, it applies to turbine engine with <<open rotor>>, wherein a free power turbine drives two contra-rotating propellers, directly or indirectly through a mechanical transmission device as a reducer and notably including a planetary gear train. In these contra-rotating propeller systems, the propellers thus have no fairing at their external radial ends.
Description of the Related Art
Turbine engine with contra-rotating propellers systems are known, propellers of which are driven by a mechanical transmission device which is usually a differential reducer. This differential reducer contains a particular planetary gear train, the sun gear of which is rotatably driven by a rotor of a free power turbine, the planet carrier of which drives the first propeller, and the ring gear of which drives the second propeller. In this respect, it is noted that depending on the position of the contra-rotating propellers with regard to the free power turbine which drives them, the first propeller constitutes the downstream propeller, and the second propeller, the upstream propeller, or vice versa. Be that as it may, unlike a simple planetary gear train, the ring gear is not stationary, but movable.
With such a planetary gear train, both propellers cannot undergo the same aerodynamic torques. The mechanical equilibrium equations of planet gears show that these two torques necessarily have a constant ratio, dependent on geometrical characteristics of the reducer. This ratio is necessarily different from the unitary ratio. Indeed, the ratio between the torque C1 applied to the first propeller and the torque C2 applied to the second propeller is given by the following:C1/C2=(R+1)/(R−1);
where R corresponds to the reducing ratio defined by the planetary gear train.
So, in order to achieve a torque ratio close to unit, it is necessary to increase the reducing ratio R, which cannot however, for questions of mechanical feasibility, be greater than 10. Furthermore, the increase of reducing ratio R is inevitably translated by an increase of the global mass of the reducer, penalizing the turbine engine.
Because of the non-unitary ratio between the torques, one of both propellers is going to generate more gyration of the bypass flow than the other propeller, which is translated by a residual gyration of the outlet flow, substantially restraining the propulsion efficiency and adversely increasing the acoustic level of the turbine engine. In fact, it is the first propeller driven by the planet carrier which is always the most loaded in terms of torque.
Besides, this difference between both couples also generates an increased stress on the means used to attach the turbine engine on the aircraft, these means being consequently oversized to withstand to the overload which is applied thereto.
By the French patent application FR 0 858 822 filed on Dec. 19, 2008 by SNECMA company, a propeller system, allowing to fill the lack of torque received by the second propeller, the second propeller being driven by the second rotor of the free power turbine, via the ring gear of the planetary gear train, is known. As a result, the aerodynamic flow is well straightened up while leaving the propeller system. Furthermore, the means used to attach the turbine engine on the aircraft are mechanically less stressed, and consequently can adopt a less expensive design in terms of footprint and mass.
In order to limit the noise of the turbine engine, the engine must include a sufficient gap between upstream and downstream propellers, which increases the length of the turbine engine. Besides, when a propeller is provided with variable setting type blades, the supply of energy (electric or hydraulic) to the setting system passes through the planetary gear train. So, any breakdown of the planetary gear train affects the setting system, which requires specific provisions to avoid hazardous conditions during the flight.